Suspension seating is used in vehicles, such as boats, to protect the riders from vibrations and impacts, especially to the spine. Studies have shown that not only the large impacts (e.g., when a boat hits large waves) can cause problems with the lower back, but also repeated low level impacts (vibrations, e.g., from chop) can have a cumulative effect on the riders, particularly regular users. Constructing a suspension mechanism that adequately protects against both forms of impact is highly problematic, since the shock absorber needs to be both soft enough at the top end to absorb small impacts whilst being stiff enough at the lower end to cope with large impacts.
A coil spring shock absorber provides a linear spring rate response and when used in the traditional manner, the position of the pivots results in a falling motion ratio, i.e. the ratio of shock strut deflection to seat deflection will become smaller as the compression stroke progresses. The rider may experience too much movement of the seat during the larger impacts and insufficient movement during the smaller impacts. The mechanism is also likely to be over-damped for the smaller impacts (and feel overly firm) and under-damped for the larger impacts. If a soft (less stiff) spring suitable for the small impacts is used, when the mechanism is subject to a large impact the spring will compress completely before the whole impact has been absorbed, maximum compression will suddenly be reached and the rider will then experience the rest of the unabsorbed impact (“bottoming out”). If on the other hand a stiffer spring is used that can absorb the energy of large impacts, it will be too stiff to absorb the small impacts adequately, imparting repeated blows to the rider which can have a cumulative effect.
A gas spring is a better option since it provides a non-linear spring rate response: initially having a low spring rate which then increases significantly towards the end of the stroke as the gas is compressed in a cylinder, i.e. a progressive spring rate. However, damping provided by the shock absorber is a function of the shock velocity. Just as the spring forces are reaching significant values to cope with the larger impacts, the shock velocity of a standard gas shock will be decreasing to a point where the damping effect is no longer felt. Unless the damping is also increased through the stroke, for example, through some modification of the shock absorber, a shock absorbing seat mechanism that provides appropriate damping for the smaller impacts at the top of the stroke will be under damped for the high spring forces experienced at the bottom of the stroke, resulting in the seat acting to eject the rider.
A typical prior art suspension arrangement utilises a parallelogram linkage arrangement. This will comprise a vehicle mount (a base) that can be bolted or otherwise secured to a floor or plinth of a vehicle, a seat support for the seat of the rider, and two links, an upper and lower link respectively, which pivotally connect the seat support to the vehicle mount. The upper and lower links are typically of the same length so that they can maintain a parallel relationship as the seat falls, keeping the seat (and the rider on the seat) in the same orientation during the suspension movement. Different manufacturers have developed different setups for the parallelogram linkage, for example that of the S2 Helmsman seat of ScotSeat KPM Marine www.scotseatkpmmarine.co.uk and that of the CoastalPro Voyager seat www.coastalpro.co.uk. Some also use other forms of suspension arrangement to link the seat support to the vehicle mount.
For the parallelogram linkage arrangements, a shock strut will typically be mounted between the vehicle mount and the seat support to absorb the shock from impacts. In general the shock strut will be arranged between the vehicle mount and seat support so that it extends approximately at right angles (between, say, 60 to 120°) to the upper and lower links. The ends of the shock strut are generally fixed with respect to the suspension seat mechanism by pivots on the seat support and vehicle mount (one end may include an adjustment mechanism to set the initial inclination of the shock strut for the weight of the rider).
During an impact, the seat support will fall with respect to the vehicle mount, following a radial path about the vehicle mount pivots for the upper and lower links. This reduces the distance between the ends of the shock strut, compressing the shock strut. During an initial part of the compression stroke, the shock strut may be inclined by, say, between 45 and 60° to the floor, and then during later parts of the compression stroke, the shock strut may become more upright as the seat continues to fall (and rotate around the vehicle mount pivots), such that it generally follows a path where it can act tangentially to resist the rotational movement of the seat support fall.
The shock strut can also be inclined in different ways, for example, substantially vertical or more inclined with the links, so long as it is subject to compression during the movement to provide a restoring force to the seat support.
While such seat suspension mechanisms using parallelogram linkages have proved popular in marine environments for a number of years now, there is significant room to improve the ride offered by them to improve comfort and reduce the cumulative effects of repeated low amplitude impacts.
Various attempts have been made to address this issue of providing a more desired spring response and damping throughout the stroke so as protect the rider against the full range of potential impacts; however they are extremely complex and expensive. Seat suspension systems are known that rely on modified shock absorbers using multiple bypass arrangements to try to optimise the ride characteristics. It is also known to provide computer controlled magneto rheological fluid systems, but the expense and complexity of such systems clearly limits their use, for example to military applications.
The solutions which have been presented so far generally reside in ways to modify how the fluids pass through the shock absorber rather than identifying new ways to utilise conventional shock struts in such mechanisms.
Accordingly, there remains a great need for a suspension system that provides adequate shock absorption and damping across a range of impacts to improve comfort and minimise the harm inflicted on riders, particularly for regular users, that is further suitable for use across a range of applications, such as (but not exclusively) in the marine environment, and that is not prohibitively expensive.
It is known to provide wheel suspension systems for the rear swing arms of motorbikes which utilise a link mechanism to guide the position of one end of the shock strut during a compression stroke in order to modify the response of the shock strut. One example is illustrated in U.S. Pat. No. 6,722,461. While the rider may appreciate the improved ride offered by the suspension system, there is no additional suspension provided between a seat support and a vehicle mount for the seat.